LIDAR system for autonomous vehicle

ABSTRACT

A method is presented for optimizing a scan pattern of a LIDAR system on an autonomous vehicle. The method includes receiving first SNR values based on values of a range of the target, where the first SNR values are for a respective scan rate. The method further includes receiving second SNR values based on values of the range of the target, where the second SNR values are for a respective integration time. The method further includes receiving a maximum design range of the target at each angle in the angle range. The method further includes determining, for each angle in the angle range, a maximum scan rate and a minimum integration time. The method further includes defining a scan pattern of the LIDAR system based on the maximum scan rate and the minimum integration time at each angle and operating the LIDAR system according to the scan pattern.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/US2019/043482 filed Jul. 25, 2019, which claims the benefit of andpriority to U.S. Patent Application No. 62/711,893, filed Jul. 30, 2018.The entire disclosures of International Application No.PCT/US2019/043482 and U.S. Patent Application No. 62/711,893 are herebyincorporated by reference as if fully set forth herein.

BACKGROUND

Optical detection of range using lasers, often referenced by a mnemonic,LIDAR, for light detection and ranging, also sometimes called laserRADAR (radio-wave detection and ranging), is used for a variety ofapplications, from altimetry, to imaging, to collision avoidance. LIDARprovides finer scale range resolution with smaller beam sizes thanconventional microwave ranging systems, such as RADAR. Optical detectionof range can be accomplished with several different techniques,including direct ranging based on round trip travel time of an opticalpulse to an object, and chirped detection based on a frequencydifference between a transmitted chirped optical signal and a returnedsignal scattered from an object, and phase-encoded detection based on asequence of single frequency phase changes that are distinguishable fromnatural signals.

To achieve acceptable range accuracy and detection sensitivity, directlong-range LIDAR systems use short pulse lasers with low pulserepetition rate and extremely high pulse peak power. The high pulsepower can lead to rapid degradation of optical components. Chirped andphase-encoded LIDAR systems use long optical pulses with relatively lowpeak optical power. In this configuration, the range accuracy increaseswith the chirp bandwidth or length and bandwidth of the phase codesrather than the pulse duration, and therefore excellent range accuracycan still be obtained.

Useful optical bandwidths have been achieved using wideband radiofrequency (RF) electrical signals to modulate an optical carrier. Recentadvances in LIDAR include using the same modulated optical carrier as areference signal that is combined with the returned signal at an opticaldetector to produce in the resulting electrical signal a relatively lowbeat frequency in the RF band that is proportional to the difference infrequencies or phases between the references and returned opticalsignals. This kind of beat frequency detection of frequency differencesat a detector is called heterodyne detection. It has several advantagesknown in the art, such as the advantage of using RF components of readyand inexpensive availability.

Recent work by current inventors, show a novel arrangement of opticalcomponents and coherent processing to detect Doppler shifts in returnedsignals that provide not only improved range but also relative signedspeed on a vector between the LIDAR system and each external object.These systems are called hi-res range-Doppler LIDAR herein. See forexample World Intellectual Property Organization (WIPO) publications WO2018/160240 and WO 2018/144853.

These improvements provide range, with or without target speed, in apencil thin laser beam of proper frequency or phase content. When suchbeams are swept over a scene, information about the location and speedof surrounding objects can be obtained. This information is expected tobe of value in control systems for autonomous vehicles, such as selfdriving, or driver assisted, automobiles.

SUMMARY

The sampling and processing that provides range accuracy and targetspeed accuracy involve integration of one or more laser signals ofvarious durations, in a time interval called integration time. To covera scene in a timely way involves repeating a measurement of sufficientaccuracy (involving one or more signals often over one to tens ofmicroseconds) often enough to sample a variety of angles (often on theorder of thousands) around the autonomous vehicle to understand theenvironment around the vehicle before the vehicle advances too far intothe space ahead of the vehicle (a distance on the order of one to tensof meters, often covered in a particular time on the order of one to afew seconds). The number of different angles that can be covered in theparticular time (often called the cycle or sampling time) depends on thesampling rate. The current inventors have recognized that a tradeoff canbe made between integration time for range and speed accuracy, samplingrate, and pattern of sampling different angles, with one or more LIDARbeams, to effectively determine the environment in the vicinity of anautonomous vehicle as the vehicle moves through that environment.

In a first set of embodiments, a method for optimizing a scan pattern ofa LIDAR system on an autonomous vehicle includes receiving, on aprocessor, data that indicates first signal-to-noise ratio (SNR) valuesof a signal reflected by a target and detected by the LIDAR system basedon values of a range of the target, where the first SNR values are for arespective value of a scan rate of the LIDAR system. The method furtherincludes receiving, on the processor, data that indicates second SNRvalues of the signal based on values of the range of the target, wherethe second SNR values are for a respective value of an integration timeof the LIDAR system. The method further includes receiving, on theprocessor, data that indicates a first angle and a second angle thatdefines an angle range of the scan pattern. The method further includesreceiving, on the processor, data that indicates a maximum design rangeof the target at each angle in the angle range. The method furtherincludes determining, for each angle in the angle range, a maximum scanrate of the LIDAR system based on a maximum value among of those scanrates where the first SNR value based on the maximum design rangeexceeds a minimum SNR threshold. The method further includesdetermining, for each angle in the angle range, a minimum integrationtime of the LIDAR system based on a minimum value among of thoseintegration times where the second SNR value based on the maximum designrange exceeds a minimum SNR threshold. The method further includesdefining, with the processor, a scan pattern of the LIDAR system basedon the maximum scan rate and the minimum integration time at each anglein the angle range. The method further includes operating the LIDARsystem according to the scan pattern.

In other embodiments, a system or apparatus or computer-readable mediumis configured to perform one or more steps of the above methods.

Still other aspects, features, and advantages are readily apparent fromthe following detailed description, simply by illustrating a number ofparticular embodiments and implementations, including the best modecontemplated for carrying out the invention. Other embodiments are alsocapable of other and different features and advantages, and theirseveral details can be modified in various obvious respects, all withoutdeparting from the spirit and scope of the invention. Accordingly, thedrawings and description are to be regarded as illustrative in nature,and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example, and not by way oflimitation, in the figures of the accompanying drawings in which likereference numerals refer to similar elements and in which:

FIG. 1A is a schematic graph that illustrates an example transmittedsignal of a series of binary digits along with returned optical signalsfor measurement of range, according to an embodiment;

FIG. 1B is a schematic graph that illustrates an example spectrum of thereference signal and an example spectrum of a Doppler shifted returnsignal, according to an embodiment;

FIG. 1C is a schematic graph that illustrates an example cross-spectrumof phase components of a Doppler shifted return signal, according to anembodiment;

FIG. 1D is a set of graphs that illustrates an example optical chirpmeasurement of range, according to an embodiment;

FIG. 1E is a graph using a symmetric LO signal, and shows the returnsignal in this frequency time plot as a dashed line when there is noDoppler shift, according to an embodiment;

FIG. 1F is a graph similar to FIG. 1E, using a symmetric LO signal, andshows the return signal in this frequency time plot as a dashed linewhen there is a nonzero Doppler shift, according to an embodiment;

FIG. 2A is a block diagram that illustrates example components of a highresolution (hi res) LIDAR system, according to an embodiment;

FIG. 2B is a block diagram that illustrates a saw tooth scan pattern fora hi-res Doppler system, used in some embodiments;

FIG. 2C is an image that illustrates an example speed point cloudproduced by a hi-res Doppler LIDAR system, according to an embodiment;

FIG. 2D is a block diagram that illustrates example components of a highresolution (hi res) LIDAR system, according to an embodiment;

FIG. 3A is a block diagram that illustrates an example system thatincludes at least one hi-res LIDAR system mounted on a vehicle,according to an embodiment;

FIG. 3B is a block diagram that illustrates an example system thatincludes at least one hi-res LIDAR system mounted on a vehicle,according to an embodiment;

FIG. 3C is a block diagram that illustrates an example of transmittedbeams at multiple angles from the LIDAR system of FIG. 3B, according toan embodiment;

FIG. 3D is a block diagram that illustrates an example system thatincludes at least one hi-res LIDAR system mounted on a vehicle,according to an embodiment;

FIG. 4A is a graph that illustrates an example signal-to-noise ratio(SNR) versus target range for the transmitted signal in the system ofFIG. 2D without scanning, according to an embodiment;

FIG. 4B is a graph that illustrates an example of a curve indicating a1/R² loss that drives the shape of the SNR curve of FIG. 4A in the farfield, according to an embodiment;

FIG. 4C is a graph that illustrates an example of collimated beamdiameter versus range for the transmitted signal in the system of FIG.2D without scanning, according to an embodiment;

FIG. 4D is a graph that illustrates an example of SNR associated withcollection efficiency versus range for the transmitted signal in thesystem of FIG. 2D without scanning, according to an embodiment;

FIG. 4E is an image that illustrates an example of beam walkoff forvarious target ranges and scan speeds in the system of FIG. 2D,according to an embodiment;

FIG. 4F is a graph that illustrates an example of coupling efficiencyversus target range for various scan rates in the system of FIG. 2D,according to an embodiment;

FIG. 4G is a graph that illustrates an example of SNR versus targetrange for various scan rates in the system of FIG. 2D, according to anembodiment;

FIG. 4H is a graph that illustrates an example of a conventional scantrajectory of the beam in the system of FIG. 2D mounted on a movingvehicle, according to an embodiment;

FIG. 4I is a graph that illustrates an example of SNR versus targetrange for various integration times in the system of FIG. 2D, accordingto an embodiment;

FIG. 4J is a graph that illustrates an example of a measurement rateversus target range in the system of FIG. 2D, according to anembodiment;

FIG. 5A is a graph that illustrates an example of a maximum design rangefor each vertical angle in the angle range in the system of FIG. 3B,according to an embodiment;

FIG. 5B is a graph that illustrates an example of integration time overthe angle range in the system of FIG. 2D, according to an embodiment;

FIG. 5C is a graph that illustrates an example of the vertical angleover time over multiple angle ranges in the system of FIG. 2D, accordingto an embodiment;

FIG. 6 is a flow chart that illustrates an example method for optimizinga scan pattern of a LIDAR system on an autonomous vehicle, according toan embodiment;

FIG. 7 is a block diagram that illustrates a computer system upon whichan embodiment of the invention may be implemented; and

FIG. 8 illustrates a chip set upon which an embodiment of the inventionmay be implemented.

DETAILED DESCRIPTION

A method and apparatus and system and computer-readable medium aredescribed for scanning of LIDAR to support operation of a vehicle. Inthe following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone skilled in the art that the present invention may be practicedwithout these specific details. In other instances, well-knownstructures and devices are shown in block diagram form in order to avoidunnecessarily obscuring the present invention.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope are approximations, the numerical values set forth inspecific non-limiting examples are reported as precisely as possible.Any numerical value, however, inherently contains certain errorsnecessarily resulting from the standard deviation found in theirrespective testing measurements at the time of this writing.Furthermore, unless otherwise clear from the context, a numerical valuepresented herein has an implied precision given by the least significantdigit. Thus a value 1.1 implies a value from 1.05 to 1.15. The term“about” is used to indicate a broader range centered on the given value,and unless otherwise clear from the context implies a broader rangearound the least significant digit, such as “about 1.1” implies a rangefrom 1.0 to 1.2. If the least significant digit is unclear, then theterm “about” implies a factor of two, e.g., “about X” implies a value inthe range from 0.5× to 2×, for example, about 100 implies a value in arange from 50 to 200. Moreover, all ranges disclosed herein are to beunderstood to encompass any and all sub-ranges subsumed therein. Forexample, a range of “less than 10” for a positive only parameter caninclude any and all sub-ranges between (and including) the minimum valueof zero and the maximum value of 10, that is, any and all sub-rangeshaving a minimum value of equal to or greater than zero and a maximumvalue of equal to or less than 10, e.g., 1 to 4.

Some embodiments of the invention are described below in the context ofa single front mounted hi-res Doppler LIDAR system on a personalautomobile; but, embodiments are not limited to this context. In otherembodiments, one or multiple systems of the same type or other highresolution LIDAR, with or without Doppler components, with overlappingor non-overlapping fields of view or one or more such systems mounted onsmaller or larger land, sea, air or space vehicles, piloted orautonomous, are employed.

1. Phase-Encoded Detection Overview

Using an optical phase-encoded signal for measurement of range, thetransmitted signal is in phase with a carrier (phase=0) for part of thetransmitted signal and then changes by one or more phase changesrepresented by the symbol Δϕ (so phase=0, Δϕ, 2Δϕ . . . ) for short timeintervals, switching back and forth between the two or more phase valuesrepeatedly over the transmitted signal. The shortest interval ofconstant phase is a parameter of the encoding called pulse duration τand is typically the duration of several periods of the lowest frequencyin the band. The reciprocal, 1/τ, is baud rate, where each baudindicates a symbol. The number N of such constant phase pulses duringthe time of the transmitted signal is the number N of symbols andrepresents the length of the encoding. In binary encoding, there are twophase values and the phase of the shortest interval can be considered a0 for one value and a 1 for the other, thus the symbol is one bit, andthe baud rate is also called the bit rate. In multiphase encoding, thereare multiple phase values. For example, 4 phase values such as Δϕ* {0,1, 2 and 3}, which, for Δϕ=π/2 (90 degrees), equals {0, π/2, π and3π/2}, respectively; and, thus 4 phase values can represent 0, 1, 2, 3,respectively. In this example, each symbol is two bits and the bit rateis twice the baud rate.

Phase-shift keying (PSK) refers to a digital modulation scheme thatconveys data by changing (modulating) the phase of a reference signal(the carrier wave). The modulation is impressed by varying the sine andcosine inputs at a precise time. At radio frequencies (RF), PSK iswidely used for wireless local area networks (LANs), RF identification(RFID) and Bluetooth communication. Alternatively, instead of operatingwith respect to a constant reference wave, the transmission can operatewith respect to itself. Changes in phase of a single transmittedwaveform can be considered the symbol. In this system, the demodulatordetermines the changes in the phase of the received signal rather thanthe phase (relative to a reference wave) itself. Since this schemedepends on the difference between successive phases, it is termeddifferential phase-shift keying (DPSK). DPSK can be significantlysimpler to implement in communications applications than ordinary PSK,since there is no need for the demodulator to have a copy of thereference signal to determine the exact phase of the received signal(thus, it is a non-coherent scheme).

For optical ranging applications, since the transmitter and receiver arein the same device, coherent PSK can be used. The carrier frequency isan optical frequency fc and a RFf₀ is modulated onto the opticalcarrier. The number N and duration τ of symbols are selected to achievethe desired range accuracy and resolution. The pattern of symbols isselected to be distinguishable from other sources of coded signals andnoise. Thus, a strong correlation between the transmitted and returnedsignal is a strong indication of a reflected or backscattered signal.The transmitted signal is made up of one or more blocks of symbols,where each block is sufficiently long to provide strong correlation witha reflected or backscattered return even in the presence of noise. Inthe following discussion, it is assumed that the transmitted signal ismade up of M blocks of N symbols per block, where M and N arenon-negative integers.

FIG. 1A is a schematic graph 120 that illustrates the exampletransmitted signal as a series of binary digits along with returnedoptical signals for measurement of range, according to an embodiment.The horizontal axis 122 indicates time in arbitrary units after a starttime at zero. The vertical axis 124 a indicates amplitude of an opticaltransmitted signal at frequency fc+f₀ in arbitrary units relative tozero. The vertical axis 124 b indicates amplitude of an optical returnedsignal at frequency fc+f₀ in arbitrary units relative to zero; and, isoffset from axis 124 a to separate traces. Trace 125 represents atransmitted signal of M*N binary symbols, with phase changes as shown inFIG. 1A to produce a code starting with 00011010 and continuing asindicated by ellipsis. Trace 126 represents an idealized (noiseless)return signal that is scattered from an object that is not moving (andthus the return is not Doppler shifted). The amplitude is reduced, butthe code 00011010 is recognizable. Trace 127 represents an idealized(noiseless) return signal that is scattered from an object that ismoving and is therefore Doppler shifted. The return is not at the properoptical frequency fc+f₀ and is not well detected in the expectedfrequency band, so the amplitude is diminished.

The observed frequency f′ of the return differs from the correctfrequency f=fc+f₀ of the return by the Doppler effect given by Equation1.

$\begin{matrix}{f^{\prime} = {\frac{\left( {c + v_{o}} \right)}{\left( {c + v_{s}} \right)}f}} & (1)\end{matrix}$Where c is the speed of light in the medium, v_(o) the velocity of theobserver and v_(s) is the velocity of the source along the vectorconnecting source to receiver. Note that the two frequencies are thesame if the observer and source are moving at the same speed in the samedirection on the vector between the two. The difference between the twofrequencies, Δf=f′-f, is the Doppler shift, Δf_(D), which causesproblems for the range measurement, and is given by Equation 2.

$\begin{matrix}{{\Delta\; f_{D}} = {\left\lbrack {\frac{\left( {c + v_{o}} \right)}{\left( {c + v_{s}} \right)} - 1} \right\rbrack f}} & (2)\end{matrix}$Note that the magnitude of the error increases with the frequency f ofthe signal. Note also that for a stationary LIDAR system (v_(o)=0), foran object moving at 10 meters a second (v_(s)=10), and visible light offrequency about 500 THz, then the size of the Doppler shift is on theorder of 16 megahertz (MHz, 1 MHz=10⁶ hertz, Hz, 1 Hz=1 cycle persecond). In various embodiments described below, the Doppler shift isdetected and used to process the data for the calculation of range.

In phase coded ranging, the arrival of the phase coded return isdetected in the return signal by cross correlating the transmittedsignal or other reference signal with the returned signal, implementedpractically by cross correlating the code for a RF signal with aelectrical signal from an optical detector using heterodyne detectionand thus down-mixing back to the RF band. Cross correlation for any onelag is computed by convolving the two traces, i.e., multiplyingcorresponding values in the two traces and summing over all points inthe trace, and then repeating for each time lag. Alternatively, thecross correlation can be accomplished by a multiplication of the Fouriertransforms of each of the two traces followed by an inverse Fouriertransform. Efficient hardware and software implementations for a FastFourier transform (FFT) are widely available for both forward andinverse Fourier transforms.

Note that the cross correlation computation is typically done withanalog or digital electrical signals after the amplitude and phase ofthe return is detected at an optical detector. To move the signal at theoptical detector to a RF frequency range that can be digitized easily,the optical return signal is optically mixed with the reference signalbefore impinging on the detector. A copy of the phase-encodedtransmitted optical signal can be used as the reference signal, but itis also possible, and often preferable, to use the continuous wavecarrier frequency optical signal output by the laser as the referencesignal and capture both the amplitude and phase of the electrical signaloutput by the detector.

For an idealized (noiseless) return signal that is reflected from anobject that is not moving (and thus the return is not Doppler shifted),a peak occurs at a time Δt after the start of the transmitted signal.This indicates that the returned signal includes a version of thetransmitted phase code beginning at the time Δt. The range R to thereflecting (or backscattering) object is computed from the two waytravel time delay based on the speed of light c in the medium, as givenby Equation 3.R=c*Δt/2  (3)

For an idealized (noiseless) return signal that is scattered from anobject that is moving (and thus the return is Doppler shifted), thereturn signal does not include the phase encoding in the properfrequency bin, the correlation stays low for all time lags, and a peakis not as readily detected, and is often undetectable in the presence ofnoise. Thus Δt is not as readily determined; and, range R is not asreadily produced.

According to various embodiments of the inventor's previous work, theDoppler shift is determined in the electrical processing of the returnedsignal; and the Doppler shift is used to correct the cross-correlationcalculation. Thus, a peak is more readily found; and, range can be morereadily determined. FIG. 1B is a schematic graph 140 that illustrates anexample spectrum of the transmitted signal and an example spectrum of aDoppler shifted complex return signal, according to an embodiment. Thehorizontal axis 142 indicates RF frequency offset from an opticalcarrier fc in arbitrary units. The vertical axis 144 a indicatesamplitude of a particular narrow frequency bin, also called spectraldensity, in arbitrary units relative to zero. The vertical axis 144 bindicates spectral density in arbitrary units relative to zero, and isoffset from axis 144 a to separate traces. Trace 145 represents atransmitted signal; and, a peak occurs at the proper RF f₀. Trace 146represents an idealized (noiseless) complex return signal that isbackscattered from an object that is moving toward the LIDAR system andis therefore Doppler shifted to a higher frequency (called blueshifted). The return does not have a peak at the proper RF f₀; but,instead, is blue shifted by Δf_(D) to a shifted frequency f_(s). Inpractice, a complex return representing both in-phase and quadrature(I/Q) components of the return is used to determine the peak at +Δf_(D),thus the direction of the Doppler shift, and the direction of motion ofthe target on the vector between the sensor and the object, is apparentfrom a single return.

In some Doppler compensation embodiments, rather than finding Δf_(D) bytaking the spectrum of both transmitted and returned signals andsearching for peaks in each, then subtracting the frequencies ofcorresponding peaks, as illustrated in FIG. 1B, it is more efficient totake the cross spectrum of the in-phase and quadrature component of thedown-mixed returned signal in the RF band. FIG. 1C is a schematic graph150 that illustrates an example cross-spectrum, according to anembodiment. The horizontal axis 152 indicates frequency shift inarbitrary units relative to the reference spectrum; and, the verticalaxis 154 indicates amplitude of the cross spectrum in arbitrary unitsrelative to zero. Trace 155 represents a cross spectrum with anidealized (noiseless) return signal generated by one object movingtoward the LIDAR system (blue shift of Δf_(D1)=Δf_(D) in FIG. 1B) and asecond object moving away from the LIDAR system (red shift of Δf_(D2)).A peak occurs when one of the components is blue shifted Δf_(D1); and,another peak occurs when one of the components is red shifted Δf_(D2).Thus, the Doppler shifts are determined. These shifts can be used todetermine a signed velocity of approach of objects in the vicinity ofthe LIDAR, as can be critical for collision avoidance applications.However, if I/Q processing is not done, peaks appear at both +/−Δ_(D1)and both +/−Δf_(D2), so there is ambiguity on the sign of the Dopplershift and thus the direction of movement.

As described in more detail in inventor's previous work the Dopplershift(s) detected in the cross spectrum are used to correct the crosscorrelation so that the peak 135 is apparent in the Doppler compensatedDoppler shifted return at lag Δt, and range R can be determined. In someembodiments simultaneous I/Q processing is performed as described inmore detail in World Intellectual Property Organization publication WO2018/144853 entitled “Method and system for Doppler detection andDoppler correction of optical phase-encoded range detection”, the entirecontents of which are hereby incorporated by reference as if fully setforth herein. In other embodiments, serial I/Q processing is used todetermine the sign of the Doppler return as described in more detail inWorld Intellectual Property Organization publication WO 2019/014177entitled “Method and System for Time Separated Quadrature Detection ofDoppler Effects in Optical Range Measurements”, the entire contents ofwhich are herby incorporated by reference as if fully set forth herein.In other embodiments, other means are used to determine the Dopplercorrection; and, in various embodiments, any method known in the art toperform Doppler correction is used. In some embodiments, errors due toDoppler shifting are tolerated or ignored; and, no Doppler correction isapplied to the range measurements.

2. Chirped Detection Overview

FIG. 1D is a set of graphs that illustrates an example optical chirpmeasurement of range, according to an embodiment. The horizontal axis102 is the same for all four graphs and indicates time in arbitraryunits, on the order of milliseconds (ms, 1 ms=10⁻³ seconds). Graph 100indicates the power of a beam of light used as a transmitted opticalsignal. The vertical axis 104 in graph 100 indicates power of thetransmitted signal in arbitrary units. Trace 106 indicates that thepower is on for a limited pulse duration, τ starting at time 0. Graph110 indicates the frequency of the transmitted signal. The vertical axis114 indicates the frequency transmitted in arbitrary units. The trace116 indicates that the frequency of the pulse increases from f₁ to f₂over the duration τ of the pulse, and thus has a bandwidth B=f₂-f₁. Thefrequency rate of change is (f₂-f₁)/τ.

The returned signal is depicted in graph 160 which has a horizontal axis102 that indicates time and a vertical axis 114 that indicates frequencyas in graph 110. The chirp 116 of graph 110 is also plotted as a dottedline on graph 160. A first returned signal is given by trace 166 a,which is just the transmitted reference signal diminished in intensity(not shown) and delayed by Δt. When the returned signal is received froman external object after covering a distance of 2R, where R is the rangeto the target, the returned signal start at the delayed time Δt is givenby 2R/c, where c is the speed of light in the medium (approximately3×10⁸ meters per second, m/s), related according to Equation 3,described above. Over this time, the frequency has changed by an amountthat depends on the range, called f_(R), and given by the frequency rateof change multiplied by the delay time. This is given by Equation 4a.f _(R)=(f ₂-f ₁)/τ*2R/c=2BR/cτ  (4a)The value of f_(R) is measured by the frequency difference between thetransmitted signal 116 and returned signal 166 a in a time domain mixingoperation referred to as de-chirping. So the range R is given byEquation 4b.R=f _(R) c τ/2B  (4b)Of course, if the returned signal arrives after the pulse is completelytransmitted, that is, if 2R/c is greater than τ, then Equations 4a and4b are not valid. In this case, the reference signal is delayed a knownor fixed amount to ensure the returned signal overlaps the referencesignal. The fixed or known delay time of the reference signal ismultiplied by the speed of light, c, to give an additional range that isadded to range computed from Equation 4b. While the absolute range maybe off due to uncertainty of the speed of light in the medium, this is anear-constant error and the relative ranges based on the frequencydifference are still very precise.

In some circumstances, a spot (pencil beam cross section) illuminated bythe transmitted light beam encounters two or more different scatterersat different ranges, such as a front and a back of a semitransparentobject, or the closer and farther portions of an object at varyingdistances from the LIDAR, or two separate objects within the illuminatedspot. In such circumstances, a second diminished intensity anddifferently delayed signal will also be received, indicated on graph 160by trace 166 b. This will have a different measured value of f_(R) thatgives a different range using Equation 4b. In some circumstances,multiple additional returned signals are received.

Graph 170 depicts the difference frequency f_(R) between a firstreturned signal 166 a and the reference chirp 116. The horizontal axis102 indicates time as in all the other aligned graphs in FIG. 1D, andthe vertical axis 164 indicates frequency difference on a much-expandedscale. Trace 176 depicts the constant frequency f_(R) measured inresponse to the transmitted chirp, which indicates a particular range asgiven by Equation4b. The second returned signal 166 b, if present, wouldgive rise to a different, larger value of f_(R) (not shown) duringde-chirping; and, as a consequence yield a larger range using Equation4b.

A common method for de-chirping is to direct both the reference opticalsignal and the returned optical signal to the same optical detector. Theelectrical output of the detector is dominated by a beat frequency thatis equal to, or otherwise depends on, the difference in the frequenciesof the two signals converging on the detector. A Fourier transform ofthis electrical output signal will yield a peak at the beat frequency.This beat frequency is in the radio frequency (RF) range of Megahertz(MHz, 1 MHz=10⁶ Hertz=10⁶ cycles per second) rather than in the opticalfrequency range of Terahertz (THz, 1 THz=10¹² Hertz). Such signals arereadily processed by common and inexpensive RF components, such as aFast Fourier Transform (FFT) algorithm running on a microprocessor or aspecially built FFT or other digital signal processing (DSP) integratedcircuit. In other embodiments, the return signal is mixed with acontinuous wave (CW) tone acting as the local oscillator (versus a chirpas the local oscillator). This leads to the detected signal which itselfis a chirp (or whatever waveform was transmitted). In this case thedetected signal would undergo matched filtering in the digital domain asdescribed in Kachelmyer 1990, the entire contents of which are herebyincorporated by reference as if fully set forth herein, except forterminology inconsistent with that used herein. The disadvantage is thatthe digitizer bandwidth requirement is generally higher. The positiveaspects of coherent detection are otherwise retained.

In some embodiments, the LIDAR system is changed to produce simultaneousup and down chirps. This approach eliminates variability introduced byobject speed differences, or LIDAR position changes relative to theobject which actually does change the range, or transient scatterers inthe beam, among others, or some combination. The approach thenguarantees that the Doppler shifts and ranges measured on the up anddown chirps are indeed identical and can be most usefully combined. TheDoppler scheme guarantees parallel capture of asymmetrically shiftedreturn pairs in frequency space for a high probability of correctcompensation.

FIG. 1E is a graph using a symmetric LO signal; and shows the returnsignal in this frequency time plot as a dashed line when there is noDoppler shift, according to an embodiment. The horizontal axis indicatestime in example units of 10⁻⁵ seconds (tens of microseconds). Thevertical axis indicates frequency of the optical transmitted signalrelative to the carrier frequency f_(c) or reference signal in exampleunits of gigaHertz (GHz, 1 GHz=10⁹ Hertz). During a pulse duration, alight beam comprising two optical frequencies at any time is generated.One frequency increases from f₁ to f₂ (e.g., 1 to 2 GHz above theoptical carrier) while the other frequency simultaneous decreases fromf₄ to f₃ (e.g., 1 to 2 GHz below the optical carrier) The two frequencybands e.g., band 1 from f₁ to f₂, and band 2 from f₃ to f₄) do notoverlap so that both transmitted and return signals can be opticallyseparated by a high pass or a low pass filter, or some combination, withpass bands starting at pass frequency f_(p). For examplef₁<f₂<f_(p)<f₃<f₄. Though, in the illustrated embodiment, the higherfrequencies provide the up chirp and the lower frequencies provide thedown chirp, in other embodiments, the higher frequencies produce thedown chirp and the lower frequencies produce the up chirp.

In some embodiments, two different laser sources are used to produce thetwo different optical frequencies in each beam at each time. However, insome embodiments, a single optical carrier is modulated by a single RFchirp to produce symmetrical sidebands that serve as the simultaneous upand down chirps. In some of these embodiments, a double sidebandMach-Zehnder intensity modulator is used that, in general, does notleave much energy in the carrier frequency; instead, almost all of theenergy goes into the sidebands.

As a result of sideband symmetry, the bandwidth of the two opticalchirps will be the same if the same order sideband is used. In otherembodiments, other sidebands are used, e.g., two second order sidebandare used, or a first order sideband and a non-overlapping secondsideband is used, or some other combination.

As described in World Intellectual Property Organization publication WO2018/160240 entitled “Method and System for Doppler Detection andDoppler Correction of Optical Chirped Range Detection,” the entirecontents of which are hereby incorporated by reference as if fully setforth herein, when selecting the transmit (TX) and local oscillator (LO)chirp waveforms, it is advantageous to ensure that the frequency shiftedbands of the system take maximum advantage of available digitizerbandwidth. In general, this is accomplished by shifting either the upchirp or the down chirp to have a range frequency beat close to zero.

FIG. IF is a graph similar to FIG. 1E, using a symmetric LO signal, andshows the return signal in this frequency time plot as a dashed linewhen there is a non-zero Doppler shift. For example, if the blue shiftcausing range effects is f_(B),then the beat frequency of the up chirpwill be increased by the offset and occur at f_(B)+Δf_(s) and the beatfrequency of the down chirp will be decreased by the offset tof_(B)-Δf_(s). Thus, the up chirps will be in a higher frequency bandthan the down chirps, thereby separating them. If Δf_(s) is greater thanany expected Doppler effect, there will be no ambiguity in the rangesassociated with up chirps and down chirps. The measured beats can thenbe corrected with the correctly signed value of the known Δf_(s) to getthe proper up-chirp and down-chirp ranges In the case of a chirpedwaveform, the time separated I/Q processing (aka time domainmultiplexing) can be used to overcome hardware requirements of otherapproaches as described above. In that case, an AOM is used to break therange-Doppler ambiguity for real valued signals. In some embodiments, ascoring system is used to pair the up and down chirp returns asdescribed in more detail in the above cited publication. In otherembodiments, I/Q processing is used to determine the sign of the Dopplerchirp as described in more detail above.

3. Optical Detection Hardware Overview

In order to depict how to use hi-res range-Doppler detection systems,some generic hardware approaches are described. FIG. 2A is a blockdiagram that illustrates example components of a high-resolution rangeLIDAR system 200, according to an embodiment. Optical signals areindicated by arrows. Electronic wired or wireless connections areindicated by segmented lines without arrowheads. A laser source 212emits a carrier wave 201 that is phase or frequency modulated inmodulator 282 a, before or after splitter 216, to produce a phase codedor chirped optical signal 203 that has a duration D. A splitter 216splits the modulated (or, as shown, the unmodulated) optical signal foruse in a reference path 220. A target beam 205, also called transmittedsignal herein, with most of the energy of the beam 201 is produced. Amodulated or unmodulated reference beam 207 a with a much smaller amountof energy that is nonetheless enough to produce good mixing with thereturned light 291 scattered from an object (not shown) is alsoproduced. In the illustrated embodiment, the reference beam 207 a isseparately modulated in modulator 282 b. The reference beam 207 a passesthrough reference path 220 and is directed to one or more detectors asreference beam 207 b. In some embodiments, the reference path 220introduces a known delay sufficient for reference beam 207 b to arriveat the detector array 230 with the scattered light from an objectoutside the LIDAR within a spread of ranges of interest. In someembodiments, the reference beam 207 b is called the local oscillator(LO) signal referring to older approaches that produced the referencebeam 207 b locally from a separate oscillator. In various embodiments,from less to more flexible approaches, the reference is caused to arrivewith the scattered or reflected field by: 1) putting a mirror in thescene to reflect a portion of the transmit beam back at the detectorarray so that path lengths are well matched; 2) using a fiber delay toclosely match the path length and broadcast the reference beam withoptics near the detector array, as suggested in FIG. 2A, with or withouta path length adjustment to compensate for the phase or frequencydifference observed or expected for a particular range; or, 3) using afrequency shifting device (acousto-optic modulator) or time delay of alocal oscillator waveform modulation (e.g., in modulator 282 b) toproduce a separate modulation to compensate for path length mismatch; orsome combination. In some embodiments, the object is close enough andthe transmitted duration long enough that the returns sufficientlyoverlap the reference signal without a delay.

The transmitted signal is then transmitted to illuminate an area ofinterest, often through some scanning optics 218. The detector array isa single paired or unpaired detector or a one-dimensional (1D) ortwo-dimensional (2D) array of paired or unpaired detectors arranged in aplane roughly perpendicular to returned beams 291 from the object. Thereference beam 207 b and returned beam 291 are combined in zero or moreoptical mixers 284 to produce an optical signal of characteristics to beproperly detected. The frequency, phase or amplitude of the interferencepattern, or some combination, is recorded by acquisition system 240 foreach detector at multiple times during the signal duration D. The numberof temporal samples processed per signal duration or integration timeaffects the down-range extent. The number or integration time is often apractical consideration chosen based on number of symbols per signal,signal repetition rate and available camera frame rate. The frame rateis the sampling bandwidth, often called “digitizer frequency.” The onlyfundamental limitations of range extent are the coherence length of thelaser and the length of the chirp or unique phase code before it repeats(for unambiguous ranging). This is enabled because any digital record ofthe returned heterodyne signal or bits could be compared or crosscorrelated with any portion of transmitted bits from the priortransmission history.

The acquired data is made available to a processing system 250, such asa computer system described below with reference to FIG. 7, or a chipset described below with reference to FIG. 8. A scanner control module270 provides scanning signals to drive the scanning optics 218,according to one or more of the embodiments described below. In oneembodiment, the scanner control module 270 includes instructions toperform one or more steps of the method 600 described below withreference to the flowchart of FIG. 6. A signed Doppler compensationmodule (not shown) in processing system 250 determines the sign and sizeof the Doppler shift and the corrected range based thereon along withany other corrections. The processing system 250 also includes amodulation signal module (not shown) to send one or more electricalsignals that drive modulators 282 a, 282 b. In some embodiments, theprocessing system also includes a vehicle control module 272 to controla vehicle on which the system 200 is installed.

Any known apparatus or system may be used to implement the laser source212, modulators 282 a, 282 b, beam splitter 216, reference path 220,optical mixers 284, detector array 230, scanning optics 218, oracquisition system 240. Optical coupling to flood or focus on a targetor focus past the pupil plane are not depicted. As used herein, anoptical coupler is any component that affects the propagation of lightwithin spatial coordinates to direct light from one component to anothercomponent, such as a vacuum, air, glass, crystal, mirror, lens, opticalcirculator, beam splitter, phase plate, polarizer, optical fiber,optical mixer, among others, alone or in some combination.

FIG. 2A also illustrates example components for a simultaneous up anddown chirp LIDAR system according to one embodiment. In this embodiment,the modulator 282 a is a frequency shifter added to the optical path ofthe transmitted beam 205. In other embodiments, the frequency shifter isadded instead to the optical path of the returned beam 291 or to thereference path 220. In general, the frequency shifting element is addedas modulator 282 b on the local oscillator (LO, also called thereference path) side or on the transmit side (before the opticalamplifier) as the device used as the modulator (e.g., an acousto-opticmodulator, AOM) has some loss associated and it is disadvantageous toput lossy components on the receive side or after the optical amplifier.The purpose of the optical shifter is to shift the frequency of thetransmitted signal (or return signal) relative to the frequency of thereference signal by a known amount Δf_(s), so that the beat frequenciesof the up and down chirps occur in different frequency bands, which canbe picked up, e.g., by the FFT component in processing system 250, inthe analysis of the electrical signal output by the optical detector230. In some embodiments, the RF signal coming out of the balanceddetector is digitized directly with the bands being separated via FFT.In some embodiments, the RF signal coming out of the balanced detectoris pre-processed with analog RF electronics to separate a low-band(corresponding to one of the up chirp or down chip) which can bedirectly digitized and a high-band (corresponding to the opposite chirp)which can be electronically down-mixed to baseband and then digitized.Both embodiments offer pathways that match the bands of the detectedsignals to available digitizer resources. In some embodiments, themodulator 282 a is excluded (e.g. in direct ranging embodiments).

FIG. 2B is a block diagram that illustrates a simple saw tooth scanpattern for a hi-res Doppler system, used in some prior art embodiments.The scan sweeps through a range of azimuth angles (horizontally) andinclination angles (vertically above and below a level direction at zeroinclination). In various embodiments described below, other scanpatterns are used. Any scan pattern known in the art may be used invarious embodiments. For example, in some embodiments, adaptive scanningis performed using methods described in World Intellectual PropertyOrganization publications WO 2018/125438 and WO 2018/102188 the entirecontents of each of which are hereby incorporated by reference as iffully set forth herein. FIG. 2C is an image that illustrates an examplespeed point cloud produced by a hi-res Doppler LIDAR system, accordingto an embodiment. Each pixel in the image represents a point in thepoint cloud which indicates range or intensity or relative speed or somecombination at the inclination angle and azimuth angle associated withthe pixel.

FIG. 2D is a block diagram that illustrates example components of ahigh-resolution (hi-res) LIDAR system 200′, according to an embodiment.In this embodiment, the system 200′ is similar to the system 200 withthe exception of the features discussed herein. In an embodiment, thesystem 200′ is a coherent LIDAR system that is constructed withmonostatic transceivers. The system 200′ includes the source 212 thattransmits the carrier wave 201 along a single-mode optical waveguideover a transmission path 222, through a circulator 226 and out a tip 217of the single-mode optical waveguide that is positioned in a focal planeof a collimating optic 219. In an embodiment, the tip 217 is positionedwithin a threshold distance (e.g. about 100 μm) of the focal plane ofthe collimating optic 219 or within a range from about 0.1% to about0.5% of the focal length of the collimating optic 219. In anotherembodiment, the collimating optic 219 includes one or more of doublets,aspheres or multi-element designs. In an embodiment, the carrier wave201 exiting the optical waveguide tip 217 is shaped by the optic 229into a collimated target beam 205′ which is scanned over a range ofangles 227 by scanning optics 218. In some embodiments, the carrier wave201 is phase or frequency modulated in a modulator 282 a upstream of thecollimation optic 229. In other embodiments, modulator 282 is excluded.In an embodiment, return beams 291 from an object are directed by thescanning optics 218 and focused by the collimation optics 229 onto thetip 217 so that the return beam 291 is received in the single-modeoptical waveguide tip 217. In an embodiment, the return beam 291 is thenredirected by the circulator 226 into a single mode optical waveguidealong the receive path 224 and to optical mixers 284 where the returnbeam 291 is combined with the reference beam 207 b that is directedthrough a single-mode optical waveguide along a local oscillator path220. In one embodiment, the system 200′ operates under the principalthat maximum spatial mode overlap of the returned beam 291 with thereference signal 207 b will maximize heterodyne mixing (opticalinterference) efficiency between the returned signal 291 and the localoscillator 207 b. This arrangement is advantageous as it can help toavoid challenging alignment procedures associated with bi-static LIDARsystems.

4. Monostatic Coherent LIDAR System Parameters

In an embodiment, monostatic coherent LIDAR performance of the system200′ is modeled by including system parameters in a so called “linkbudget” and a LIDAR is operated based on that modeling. A link budgetestimates the expected value of the signal to noise ratio (SNR) forvarious system and target parameters. In one embodiment, on the systemside, a link budget includes one or more of output optical power,integration time, detector characteristics, insertion losses inwaveguide connections, mode overlap between the imaged spot and themonostatic collection waveguide, and optical transceivercharacteristics. In another embodiment, on the target side, a linkbudget includes one or more of atmospheric characteristics, targetreflectivity, and target range.

FIG. 4A is a graph that illustrates an example signal-to-noise ratio(SNR) versus target range for the return beam 291 in a LIDAR systemwithout scanning, such as the system 200′ of FIG. 2D or the system 200of FIG. 2A. The horizontal axis 402 is target range in units of meters(m). The vertical axis 404 is SNR in units of decibels (dB). A trace 410depicts the values of SNR versus range that is divided into a near field406 and a far field 408 with a transition from the near field 406 of thecurve 410 with a relatively flat slope to the far field 408 of the curve410 with a near constant negative slope (e.g. about −20 dB per 10 m).The reduction in SNR in the far field 408 is dominated by “r-squared”losses, since the scattering atmosphere through which the return beam291 passes grows with the square of the range to the target while thesurface area of the optical waveguide tip 217 to collect the return beam291 is fixed. FIG. 4B is a graph that illustrates an example of a trace411 indicating 1/R²loss that drives the shape of the SNR curve 410 inthe far field 408, according to an embodiment. The horizontal axis 402is range in units of meters (m) and the vertical axis 407 indicate powerloss that is unitless on a logarithmic scale.

In the near field 406, a primary driver of the SNR is a diameter of thecollimated return beam 291 before it is focused by the collimationoptics 229 to the tip 217. FIG. 4C is a graph that illustrates anexample of collimated beam diameter versus range for the return beam 291in the system 200′ of FIG. 2D without scanning, according to anembodiment. The horizontal axis 402 indicates target range in units ofmeters (m) and the vertical axis 405 indicates diameter of the returnbeam 291 in units of meters (m). In an embodiment, curve 414 depicts thediameter of the collimated return beam 291 incident on the collimationoptics 229 prior to the return beam 291 being focused to the tip 217 ofthe optical waveguide. The trace 414 illustrates that the diameter ofthe collimated return beam 291 incident on the collimation optics 229increases with increasing target range.

In an embodiment, in the near field 406, as the diameter of thecollimated return beam 291 grows at larger target ranges, a diameter ofthe focused return beam 291 by the collimation optics 229 at the tip 217shrinks. FIG. 4D is a graph that illustrates an example of SNRassociated with collection efficiency of the return beam 291 at the tip217 versus range for the transmitted signal in the system of FIG. 2Dwithout scanning, according to an embodiment. The horizontal axis 402indicates target range in units of meters (m) and the vertical axis 404indicates SNR in units of decibels (dB). The trace 416 depicts the nearfield SNR of the focused return beam 291 by the collimation optics 229at the tip 217 based on target range. At close ranges within the nearfield 406, an image 418 a of the focused return beam 291 at the tip 217by the collimation optics 229 is sufficiently larger than the core sizeof the single mode optical fiber tip 217. Thus, the tip receives less ofthe returned signal; and, the SNR associated with the collectionefficiency is relatively low. At longer ranges within the near field406, an image 418 b of the focused return beam 291 at the tip 217 by thecollimation optics 229 is much smaller than the image 418 a and thus theSNR attributable to the collection efficiency increases at longerranges. In an embodiment, the curve 416 demonstrates that the SNR innear field 406 has a positive slope (e.g. +20 dB per 10 meters) based onthe improved collection efficiency of the focused return beam 291 atlonger ranges. In one embodiment, this positive slope in the near fieldSNR cancels the negative slope in the near field SNR discussed in FIG.4B that is attributable to “r-squared” losses and thus leads to therelatively flat region of the SNR curve 410 in the near field 406. Thepositive slope in the SNR curve 416 in FIG. 4D does not extend into thefar field 408 and thus the “r-squared” losses of FIG. 4B dominate thefar field 408 SNR as depicted in the SNR curve 410 in the far field 408.

While the discussion in relation to FIGS. 4A-4D predicts SNR of thereturn beam 291 as a function of the target range, the predicted SNR inFIGS. 4A-4D does not fully characterize the performance of the scannedmonostatic coherent LIDAR system 200′ since it does not consider a scanrate of the scanning optics 218. In an embodiment, due to round tripdelay of the return beam 291, the receive mode of the return beam 291will laterally shift or “walk off” from the transmitted mode of thetransmitted beam 205′ when the beam is being scanned by the scanningoptics 218. FIG. 4E is an image that illustrates an example of beamwalkoff for various target ranges and scan speeds in the system 200′ ofFIG. 2D, according to an embodiment. The horizontal axis 402 indicatestarget range and the vertical axis 422 indicates scan speed of the beamusing the scanning optics 218. As FIG. 4E depicts, there is no beamwalkoff when the beam is not scanned (bottom row) since the image 418 aof the focused return beam 291 is centered on the fiber tip 217demonstrating no beam walkoff at short target range; and the image 418 bof the focused return beam 291 is also centered on the fiber tip 217demonstrating no beam walkoff at far target range. When the beam isscanned at a moderate scan speed (middle row in FIG. 4E), a moderatebeam walkoff 419 a is observed between the image 418 a of the focusedreturn beam 291 and the fiber tip 217 and a larger beam walkoff 419 b isobserved between the image 418 b of the focused return beam 291 and thefiber tip 217. The loss of intensity is relative less in the near fieldand relatively more in the far field. When the beam is scanned at a highscan speed (top row in FIG. 4E), a beam walkoff 421 a is observed atshort range that exceeds the beam walkoff 419 a at the moderate scanspeed; and a beam walkoff 421 b is observed at large range that exceedsthe beam walk off 419 b at the moderate scan speed. Thus, the beamwalkoff increases as the target range and scan speed increase. In anembodiment, increased target range induces a time delay during which theimage 418 a, 418 b shifts away from the tip 217 of the fiber core. Thus,a model of the mode overlap accounts for this walkoff appropriately, andoperation of the LIDAR is adapted to account for this effect. In oneembodiment, such operation should limit the beam walkoff 419 based on adiameter of the image 418 (e.g., no greater than half of the diameter ofthe image 418).

FIG. 4F is a graph that illustrates an example of coupling efficiencyversus target range for various scan rates in the system 200′ of FIG.2D, according to an embodiment. The horizontal axis 402 indicates targetrange in units of meters (m) and the vertical axis 430 indicatescoupling efficiency which is unitless. In the illustrated embodiment,the coupling efficiency is inversely proportional to the beam walkoff419. A first trace 432 a depicts the coupling efficiency of the focusedreturn beam 291 into the fiber tip 217 for various target ranges basedon no scanning of the beam. The coupling efficiency remains relativelyhigh and constant for a wide range of target ranges. A second trace 432b depicts the coupling efficiency of the focused return beam 291 intothe fiber tip 217 for various target ranges based on moderate scan rateof the beam. In this embodiment, the coupling efficiency at the moderatescan rate peaks at a moderate target range (e.g. about 120 m) and thendecreases as target range increases. A third trace 432 c depicts thecoupling efficiency of the focused return beam 291 into the fiber tip217 for various target ranges based on a high scan rate of the beam. Inthis embodiment, the coupling efficiency of the high scan rate peaks ata low target range (e.g. about 80 m) and then decreases as target rangeincreases.

Based on the traces in FIG. 4F, scanning too fast will eventually makeit impossible to see beyond some target range. In this instance, theimage 418 b of the focused return beam 291 does not couple into thefiber tip 217 and instead has totally walked off the receiver mode ofthe tip 217. FIG. 4G is a graph that illustrates an example of SNRversus target range for various scan rates in the system 200′ of FIG.2D, according to an embodiment. The horizontal axis 402 indicates targetrange in units of meters (m) and the vertical axis 404 indicates SNR inunits of decibels (dB). A first trace 440 a depicts the SNR of thefocused return beam 291 on the fiber tip 217 based on target range wherethe beam is not scanned. A second trace 440 b depicts the SNR of thefocused return beam 291 on the fiber tip 217 based on target range wherethe beam is scanned at a moderate scan rate. In an example embodiment,the moderate scan rate is about 2500 degrees per sec (deg/sec) or in arange from about 1000 deg/sec to about 4000 deg/sec or in a range fromabout 500 deg/sec to about 5000 deg/sec. A third trace 440 c depicts theSNR of the focused return beam 291 on the fiber tip 217 based on targetrange where the beam is scanned at a high scan rate. In an exampleembodiment, the high scan rate is about 5500 deg/sec or in a range fromabout 4000 deg/sec to about 7000 deg/sec or in a range from about 3000deg/sec to about 8000 deg/sec. In an embodiment, the moderate scan rateand high scan rate are based on a beam size and goal of the system. Inan example embodiment, the numerical ranges of the moderate scan rateand high scan rate above are based on a collimated beam with a diameterof about 1 centimeter (cm) used to scan an image out to a maximum targetrange of about 200 meters (m).

Ultimately, beam walk off 419 is a significant inhibitor of SNR incoherent monostatic LIDAR system 200′. In conventional coherentmonostatic LIDAR systems, the scan rate of the beam is set at a fixedscan rate over the angle range and the resulting target range, where thefixed scan rate is chosen so that the associated SNR of the fixed scanrate is above an SNR threshold for the entire target range of interest.In conventional coherent LIDAR systems, this results in a relatively lowfixed scan rate being used to scan the beam over a scan trajectory 460,which results in large gaps 462 between adjacent scans, as depicted inFIG. 4H. The scan speed limitation leads to dense sampling along thebeam trajectory 460. When the beam is scanned over a reasonably largefield of few (e.g. 10's of degrees in either dimension), the beamtrajectory 460 leaves large gaps 462 in the angular coverage. This isnot ideal as some targets positioned in the large gaps 462 can goundetected. A “square grid” of rectangular sampling is not achieved.Instead, an asymmetry is observed between the sampling along the scantrajectory 460 and the gaps 462 between the trajectory 460, which can begreater than 10:1. With this problem in mind, the inventors of thevarious embodiments developed several complementary solutions includingone or more of maximizing beam scan speed when possible, directly “stepscan” the beam between discrete locations, and generate efficienthardware solutions for these concepts.

In addition to the scan rate of the beam, the SNR of the return beam 291is affected by the integration time over which the acquisition system240 and/or processing system 250 samples and processes the return beam291. In some embodiments, the beam is scanned between discrete anglesand is held stationary or almost stationary at discrete angles in theangle range 227 for a respective integration time at each discreteangle. The SNR of the return beam 291 is affected by the value of theintegration time and the target range. As previously discussed, thecross-sectional area of the beam increases with target range resultingin increased atmospheric scattering and thus an intensity of the returnbeam 291 decreases with increasing range at the rate of 1/R².Accordingly, a longer integration time is needed to achieve the same SNRfor a return beam 291 from a longer target range.

FIG. 4I is a graph that illustrates an example of SNR versus targetrange for various integration times in the system 200′ of FIG. 2D,according to an embodiment. The horizontal axis 402 indicates targetrange in units of meters (m) and the vertical axis 404 indicates SNR inunits of decibels (dB). A first trace 450 a depicts SNR values of thereturn beam 291 over the target range, where the system 200′ is set to afirst integration time (e.g., 3.2 μs). A second trace 450 b depicts SNRvalues of the return beam 291 over the target range, where the system200′ is set to a second, shorter integration time (e.g., 1.6 μs). Athird trace 450 c depicts SNR values of the return beam 291 over thetarget range, where the system 200′ is set to a third even shorterintegration time (e.g., 800 nanoseconds, ns, 1 ns=10⁻⁹ seconds). Afourth trace 450 d depicts SNR values of the return beam 291 over thetarget range, where the system 200′ is set to a fourth still shorterintegration time (e.g., 400 ns). The traces 450 a through 450 d,collectively referenced hereinafter as traces 450, demonstrate that fora fixed target range, an increased SNR is achieved with increasingintegration time. The traces 450 also demonstrate that for a fixedintegration time, the SNR of the return beam 291 decreases withincreased range, for the reasons previously discussed.

Conventional monostatic LIDAR systems typically select a fixedintegration time (e.g., 1.6 μs) for the scanning at the range of angles227 and resulting target ranges, so that the SNR associated with thefixed integration time exceeds a target SNR threshold indicated byhorizontal dashed line 452 over the target range. It is here recognizedthat this results in relatively high integration times being used toscan the beam over the range of angles 227 in conventional monostaticLIDAR systems, integration times that are higher than needed at someangles.

Here is presented a method for operating a LIDAR that addresses thisnoted drawback, including minimizing the integration time at each anglewithin the range of angles 227 using the target range at each angle, soto minimize the integration time over at least some angles 227. FIG. 4Jis a graph that illustrates an example of a measurement rate versustarget range used to operate a LIDAR, such as the system 200′ of FIG.2D, according to an embodiment. The horizontal axis 402 indicates targetrange in units of meters (m) and the vertical axis 474 indicates numberof allowable measurements per unit time in units of number per second.Trace 476 depicts the number of allowable measurements per second ateach target range. In an embodiment, trace 476 represents an inverse ofthe integration time, e.g. the number of return beams 291 that can bedetected at each target range per second whereas integration timeconveys how long it takes to process the return beam 291 at each targetrange. Trace 478 depicts a receipe for operating a LIDAR for a discretenumber of allowable measurements per second at each of various targetrange steps. The trace 478 is based on power of 2 intervals for a givenADC (analog to digital conversion) rate. An advantage of trace 478 isthat the number of digitized samples is a power of 2; and, thus, adigital signal processing implementation of a fast Fourier transform onsuch a length signal is more efficient.

5. Vehicle Control Overview

In some embodiments a vehicle is controlled at least in part based ondata received from a hi-res Doppler LIDAR system mounted on the vehiclewhen the LIDAR is operated as described herein.

FIG. 3A is a block diagram that illustrates an example system 301 thatincludes at least one hi-res Doppler LIDAR system 320 mounted on avehicle 310, according to an embodiment. In some embodiments, the LIDARsystem 320 is similar to one of the LIDAR systems 200, 200′. The vehiclehas a center of mass indicted by a star 311 and travels in a forwarddirection given by arrow 313. In some embodiments, the vehicle 310includes a component, such as a steering or braking system (not shown),operated in response to a signal from a processor, such as the vehiclecontrol module 272 of the processing system 250. In some embodiments,the vehicle has an on-board processor 314, such as chip set depicted inFIG. 8. In some embodiments, the on-board processor 314 is in wired orwireless communication with a remote processor, as depicted in FIG. 7.In an embodiment, the processing system 250 of the LIDAR system 200 iscommunicatively coupled with the on-board processor 314; or theprocessing system 250 of the LIDAR is used to perform the operations ofthe on-board processor 314 so that the vehicle control module 272 causesthe processing system 250 to transmit one or more signals to thesteering or braking system of the vehicle to control the direction andspeed of the vehicle. The hi-res Doppler LIDAR uses a scanning beam 322that sweeps from one side to another side, represented by future beam323, through an azimuthal field of view 324, as well as through verticalangles (326 in FIG. 3B), illuminating spots in the surroundings ofvehicle 310. In some embodiments, the field of view is 360 degrees ofazimuth. In some embodiments the inclination angle field of view is fromabout +10 degrees to about −10 degrees or a subset thereof.

In some embodiments, the vehicle includes ancillary sensors (not shown),such as a GPS sensor, odometer, tachometer, temperature sensor, vacuumsensor, electrical voltage or current sensors, among others well knownin the art. In some embodiments, a gyroscope 330 is included to providerotation information.

FIG. 3B is a block diagram that illustrates an example system 301′ thatincludes at least one hi-res LIDAR system 320 mounted on the vehicle310, according to an embodiment. In an embodiment, the LIDAR system 320is similar to the system 200 or system 200′. In one embodiment, thevehicle 310 moves over the surface 349 (e.g., road) with the forwarddirection based on the arrow 313. The LIDAR system 320 scans over arange of angles 326 from a first beam 342 oriented at a first anglemeasured with respect to the arrow 313 to a second beam 346 oriented ata second angle measured with respect to the arrow 313. In oneembodiment, the first angle and the second angle are vertical(inclination) angles within a vertical plane that is oriented aboutorthogonal with respect to the surface 349. For purposes of thisdescription, “about orthogonal” means within ±20 degrees of a normal tothe surface 349.

In designing the system 301′, a predetermined maximum design range ofthe beams at each angle is determined and represents a maximumanticipated target range at each angle in the range 326. In otherembodiments, the maximum design range at each angle is not predeterminedbut is regularly measured and updated in the memory of the processingsystem 250 at incremental time periods. In an embodiment, the first beam342 is oriented toward the surface 349 (inclined downward) andintersects the surface 349 within some maximum design range from thevehicle 310. Thus, at the first angle, the system 320 does not considertargets positioned below the surface 349. In an example embodiment, thefirst angle of the first beam 342 is about −15 degrees or in a rangefrom about −25 degrees to about −10 degrees with respect to the arrow313; and the maximum design range is about 4 meters (m) or within arange from about 1 m to about 10 m, or in a range from about 2 m toabout 6 m. In an embodiment, the second beam 346 is inclined upward(e.g., oriented toward the sky) and intersects a ceiling 347 above whichthe operation of vehicle 310 is not concerned, within some maximumdesign range from the vehicle 310. Thus, at the second angle the system320 does not consider targets positioned above the ceiling 347. In anexample embodiment, the ceiling 347 is at an altitude of about 12 m orin a range from about 8 m to about 15 m from the surface 349 (e.g., thatdefines an altitude of 0 m), the second angle of the second beam 346 isabout 15 degrees or in a range from about 10 degrees to about 20 degreeswith respect to the arrow 313; and the maximum design range is about 7 mor within a range from about 4 m to about 10 m or within a range fromabout 1 m to about 15 m. In some embodiments, the ceiling 347 altitudedepends on an altitude of the LIDAR system 320 (e.g. about 1.5 m or in arange of about 1 m to about 4 m, where the surface 349 defines 0 m). Inan embodiment, an intermediate beam 344 between the first beam 342 andsecond beam 346 is oriented about parallel with the arrow 313 andintersects a target 343 positioned at a maximum design range from thevehicle 310. FIG. 3B is not drawn to scale and in various embodimentstarget 343 is positioned at a much further distance from the vehicle 310than appears in FIG. 3B. For purposes of this description, “aboutparallel” means within about ±10 degrees or within about ±15 degrees ofthe arrow 313. In an example embodiment, the maximum design range of thetarget 343 is about 200 m, or within a range from about 150 m to about300 m, or within a range from about 100 m to about 500 m, or within arange of about 100 m to about 1000 m or more.

FIG. 5A is a graph that illustrates an example of the predeterminedmaximum design range for each vertical angle in the angle range in thesystem 320 of FIG. 3B, according to an embodiment. The horizontal axis502 indicates target range in units of meters (m) and the vertical axis510 indicates angle of the beam in units of degrees (deg). Trace 512 adepicts the maximum design range for those beams with an upwardinclination in the range of angles 326 with a positive angle withrespect to arrow 313 (e.g., beam 346). Trace 512 b depicts the maximumdesign range for those beams with a downward inclination in the range ofangles 326 with a negative angle with respect to arrow 313 (e.g. beam342).

Although FIG. 3B depicts the LIDAR system mounted on a vehicle 310configured to travel over the surface 349, the embodiments of thepresent is not limited to this type of vehicle and the LIDAR system canbe mounted to an air vehicle 310′ (e.g. passenger air vehicle) that isconfigured to fly. In an embodiment, the vehicle 310′ is configured tofly over a surface 349 where one or more targets 343 are present. FIG.3D is a block diagram that illustrates an example system 301″ thatincludes at least one hi-res LIDAR system 320 mounted on a vehicle 310′configured to fly over a surface 349, according to an embodiment. In anembodiment, the LIDAR system 320 operates in a similar manner as theLIDAR system 320 of the system 301′, with the exception that the maximumdesign range of the first beam 342′ at the first angle with respect tothe arrow 313 is defined based on a floor 348 relative to the surface349. In an example embodiment, the floor 348 has an altitude relative tothe altitude of the system 320 in a range from about 0 m to about −10 mor in a range from about 0 m to about −2 m. In another exampleembodiment, the ceiling 347 has an altitude relative to the altitude ofthe system 320 in a range from about 0 m to about 10 m. In anotherexample embodiment, the first angle is about −30 degrees or within arange from about −60 degrees to about −15 degrees. In some embodiments,the first angle would be equal and opposite to the second angle of theceiling 347.

6. Method for Optimization of Scan Pattern in Coherent LIDAR System

FIG. 6 is a flow chart that illustrates an example method 600 foroptimizing a scan pattern of a LIDAR system on an autonomous vehicle.Although steps are depicted in FIG. 6, and in subsequent flowcharts asintegral steps in a particular order for purposes of illustration, inother embodiments, one or more steps, or portions thereof, are performedin a different order, or overlapping in time, in series or in parallel,or are omitted, or one or more additional steps are added, or the methodis changed in some combination of ways.

In step 601, data is received on a processor that indicates first SNRvalues of a signal reflected by a target and detected by the LIDARsystem based on values of a range of the target, where the first SNRvalues are for a respective value of a scan rate of the LIDAR system. Inan embodiment, in step 601 the data is first SNR values of the focusedreturn beam 291 on the fiber tip 217 in the system 200′. In anotherembodiment, in step 601 the data is first SNR values of the focusedreturn beam 291 on the detector array 230 in the system 200. In oneembodiment, the data includes values of traces 440 a and/or trace 440 band/or trace 440 c, or the equations or models that produce thosetraces, and which indicate SNR values of the return beam 291, where eachtrace 440 is for a respective value of the scan rate of the beam. Insome embodiments, the data is not limited to traces 440 a, 440 b, 440 cand includes SNR values of less or more curves than are depicted in FIG.4G, where each SNR curve is based on a respective value of the scan rateand actual configuration of collimators and return tips or the equationsor models that produce those traces. In other embodiments, the dataincludes SNR values that could be used to form the curve over the targetrange for each respective value of the scan rate. In an exampleembodiment, in step 601 the data is stored in a memory of the processingsystem 250 and each set of first SNR values is stored with an associatedvalue of the scan rate of the LIDAR system. In one embodiment, in step601 the first SNR values are obtained over a range from about 0 metersto about 500 meters (e.g. automotive vehicles) or within a range fromabout 0 meters to about 1000 meters (e.g. airborne vehicles) and forscan rate values from about 2000 deg/sec to about 6000 deg/sec or withina range from about 1000 deg/second to about 7000 deg/sec

In some embodiments, the first SNR values are predetermined and arereceived by the processor in step 601. In other embodiments, the firstSNR values are measured by the LIDAR system and subsequently received bythe processor in step 601. In one embodiment, the data is input in step601 using an input device 712 and/or uploaded to the memory 704 of theprocessing system 250 over a network link 778 from a local area network780, internet 790 or external server 792, either unsolicited or inresponse to a query.

In step 603, data is received on a processor that indicates second SNRvalues of a signal reflected by a target and detected by the LIDARsystem based on values of a range of the target, where the second SNRvalues are for a respective value of an integration time of the LIDARsystem. In an embodiment, in step 603 the data is second SNR values ofthe focused return beam 291 in the system 200′ for a respectiveintegration time over which the beam is processed by the acquisitionsystem 240 and/or processing system 250. In one embodiment, the dataincludes values of trace 450 a and/or trace 450 b and/or trace 450 cand/or trace 450 d or the equations or models that produce those traces,and which indicate SNR values of the return beam 291, where each trace450 is for a respective value of the integration time that the beam isprocessed by the acquisition system 240 and/or processing system 250. Insome embodiments, the data is not limited to traces 450 a, 450 b, 450 c,450 d and includes less or more traces than are depicted in FIG. 4I,where each SNR curve is based on a respective value of the integrationtime or the configuration of collimators and sensor tips, or theequations or models that produce those traces. In other embodiments, thedata need not be a trace and instead is the SNR values used to form thetrace over the target range for each respective value of the integrationtime. In an example embodiment, in step 603 the data is stored in amemory of the processing system 250 and each set of second SNR values isstored with an associated value of the integration time of the LIDARsystem. In one embodiment, in step 603 the second SNR values areobtained over a range from about 0 meters to about 500 meters (e.g.automotive vehicles) or from a range from about 0 meters to about 1000meters (e.g. airborne vehicles) and for integration time values fromabout 100 nanosecond (ns) to about 5 microseconds (μs).

In some embodiments, the second SNR values are predetermined and arereceived by the processor in step 603. In other embodiments, the secondSNR values are measured by the LIDAR system and subsequently received bythe processor in step 603. In one embodiment, the data is input in step603 using an input device 712 and/or uploaded to the memory 704 of theprocessing system 250 over a network link 778 from a local area network780, internet 790 or external server 792, either unsolicited or inresponse to a query.

In step 605, data is received on a processor that indicates the firstangle and the second angle that defines the angle range 326. In oneembodiment, in step 605 the first angle and the second angle are inputusing an input device 712 (e.g. mouse or pointing device 716) and/or areuploaded to the processing system 250 over a network link 778. In anembodiment the first angle is defined as the angle between the firstbeam 342 and the arrow 313 indicating the direction of travel of thevehicle 310 and the second angle is defined as the angle between thesecond beam 346 and the arrow 313. In an embodiment, the first angle andsecond angle are symmetric with respect to the arrow 313, e.g. the firstangle and the second angle are equal and opposite to each other. In oneembodiment, the first angle is selected so that the first beam 342 isoriented towards the surface 349 and the second angle is selected sothat the second beam 346 is oriented away from the surface 349 andtowards the ceiling 347, in one or two dimensions. In one embodiment,steps 601, 603 and 605 are simultaneously performed in one step wherethe data in steps 601, 603 and 605 is received at the processor in onesimultaneously step.

In step 607, data is received on a processor that indicates the maximumdesign range of the target at each angle in the angle range. In anembodiment, the maximum design range is the predetermined maximum rangeof the target at each angle in the range of angles 326. In oneembodiment, in step 607 the maximum design range of the target at eachangle is provided by the traces 512 a, 512 b. The maximum design rangedata received in step 607 is not limited to the data of the traces 512a, 512 b which is based on a specific range of angles 326. In oneembodiment, traces 512 a, 512 b depict that the maximum design range forbeams 342, 346 at the first angle and the second angle (e.g. <100 m) isless than the maximum design range of intermediate beam 344 at an anglebetween the first and second angle (e.g. >100 m). In another embodiment,the data in step 607 is provided over a first range of angles that isgreater than the range of angles 326. In one embodiment, the data instep 607 is provided at an incremental angle over the angle range,wherein the incremental angle is selected in a range from about 0.005degrees to about 0.01 degrees or in a range from about 0.0005 degrees toabout 0.01 degrees.

In one example embodiment, the data in step 607 is input using an inputdevice 712 (e.g. mouse or pointing device 716) and/or are uploaded tothe processing system 250 over a network link 778. In some embodiments,the maximum design range is predetermined and received during step 607.In other embodiments, the system 200, 200′ is used to measure themaximum design range at each angle in the angle range 326 and themaximum design range at each angle is subsequently received by theprocessing system 250 in step 607.

In step 609, a maximum scan rate of the LIDAR system is determined ateach angle in the angle range 326 so that the SNR of the LIDAR system isgreater than a minimum SNR threshold. At each angle, the maximum designrange for that angle is first determined based on the received data instep 607. In one embodiment, the maximum design range for the angle isdetermined using trace 512 a or 512 b. First SNR values received in step601 are then determined for the maximum design range at the angle and itis further determined which of these first SNR values exceed the minimumSNR threshold. In one embodiment, values of traces 440 a, 440 b, 440 care determined for a maximum design range (e.g. about 120 m) and it isfurther determined that the values of traces 440 a, 440 b exceeds theminimum SNR threshold 442. Among those first SNR values which exceed theminimum SNR threshold, the first SNR values with the maximum scan rateis selected and the maximum scan rate is determined in step 609 for thatangle. In the above embodiment, among the values of the traces 440 a,440 b which exceeds the minimum SNR threshold 442 at the maximum designrange (e.g. about 120 m), the curve 440 b values are selected as themaximum scan rate and the maximum scan rate (e.g. moderate scan rateassociated with curve 440 b) is determined in step 609 for that angle.In an embodiment, FIG. 4G depicts that the maximum scan rate determinedin step 609 for the beams 342, 346 at the first and second angle (e.g.fast scan rate based on curve 440 c) is greater than the maximum scandetermined in step 609 for beam 344 at an angle between the first andsecond angle (e.g. moderate scan rate based on curve 440 b). In anexample embodiment, the determining of the maximum scan rate in step 609ensures that beam walkoff 419 (FIG. 4E) of the return beam 291 on thefiber tip 217 is less than a ratio of a diameter of the image 418 of thereturn beam 291 on the tip 217. In an example embodiment, the ratio isabout 0.5 or in a range from about 0.3 to about 0.7.

FIG. 3C is a block diagram that illustrates an example of transmittedbeams 343 a, 343 b at multiple angles 345 a, 345 b from the LIDAR system320 of FIG. 3B, according to an embodiment. In one embodiment, beams 343a, 343 b are intermediate beams between first beam 342 and intermediatebeam 344. In other embodiments, beam 343 a is the first beam 342 andbeam 343 b is a subsequent beam that is processed after the first beam342. In step 609, the maximum scan rate of the LIDAR system 320 isdetermined at the angle 345 a. The maximum design range (e.g. 30 m) ofthe beam 343 a at the angle 345 a is first determined using the data instep 607. First SNR values from step 601 for the maximum design rangeare then determined. In an example embodiment, the first SNR valuesinclude values of traces 440 a, 440 b, 440 c. It is then determinedwhich of those first SNR values at the maximum design range exceeds theminimum SNR threshold. In the example embodiment, the values of thetraces 440 a, 440 b, 440 c at the maximum design range (e.g. 30 m) allexceed the minimum SNR threshold 442. It is then determined which ofthese first SNR values has the maximum scan rate and this maximum scanrate is determined in step 609 for that angle. In the exampleembodiment, the curve 440 c has the maximum scan rate and thus thismaximum scan rate is used to scan the beam 343 a at the angle 345 a. Inan embodiment, FIG. 4I depicts that the minimum scan rate determined instep 611 for the beams 342, 346 at the first and second angle (e.g. 400ns based on curve 450 d) is shorter than the minimum integration timedetermined in step 611 for beam 344 at an angle between the first andsecond angle (e.g. 3.2 μs based on curve 450 a).

In step 611, a minimum integration time of the LIDAR system isdetermined at each angle in the angle range 326 so that the SNR of theLIDAR system is greater than a minimum SNR threshold. At each angle, themaximum design range for that angle is first determined based on thereceived data in step 607. In one embodiment, the maximum design rangefor the angle is determined using trace 512 a or 512 b. Second SNRvalues received in step 603 are then determined for the maximum designrange at the angle and it is further determined which of these secondSNR values exceed the minimum SNR threshold. In one embodiment, valuesof traces 450 a, 450 b, 450 c, 450 d are determined for a maximum designrange (e.g. about 120 m) and it is further determined that the values oftraces 450 a, 450 b, 450 c exceeds the minimum SNR threshold 452. Amongthose second SNR values which exceed the minimum SNR threshold, thesecond SNR values with the minimum integration time is selected and theminimum integration time is determined in step 611 for that angle. Inthe above embodiment, among the values of the curves 450 a, 450 b, 450 cwhich exceeds the minimum SNR threshold 452 at the maximum design range(e.g. about 120 m), the curve 450 c values are selected with the minimumintegration time and the minimum integration time (e.g. about 800 ns) isdetermined in step 611 for that angle.

In step 611, the minimum integration time of the LIDAR system 320 isdetermined at the angle 345 a. The maximum design range (e.g. 30 m) ofthe beam 343 a at the angle 345 a is first determined using the data instep 607. Second SNR values from step 603 for the maximum design rangeare then determined. In an example embodiment, the second SNR valuesinclude values of tracess 450 a, 450 b, 450 c, 450 d. It is thendetermined which of those second SNR values at the maximum design rangeexceeds the minimum SNR threshold. In the example embodiment, the valuesof the traces 450 a, 450 b, 450 c, 450 d at the maximum design range(e.g. 30 m) all exceed the minimum SNR threshold 452. It is thendetermined which of these second SNR values has the minimum integrationtime and this minimum integration time is determined in step 611 forthat angle. In the example embodiment, the curve 450 d has the minimumintegration time (e.g. about 400 ns) and thus this minimum integrationtime is used to process the beam 343 a at the angle 345 a.

In step 613, a determination is made whether additional angles remain inthe angle range 326 to perform another iteration of steps 609, 611. Inone embodiment, where an initial iteration of steps 609, 611 was at thefirst angle of the first beam 342 in the range 326, step 613 involves adetermination of whether the previous iteration of steps 609, 611 was ator beyond the second angle of the second beam 346 in the angle range326. In another embodiment, where an initial iteration of the steps 609,611 was at the second angle of the range 326, step 613 involves adetermination of whether the previous iteration of steps 609, 611 was ator beyond the first angle of the angle range 326. If step 613 determinesthat more angles in the range 326 remain, the method proceeds to block615. If step 613 determines that no further angles in the range 326remain, the method proceeds to block 617.

In step 615, a determination is made of a subsequent angle at which toiterate steps 609, 611 after having performed a previous iteration ofsteps 609, 611 at an initial angle. In an embodiment, FIG. 3C depicts asubsequent angle 345 b of the subsequent beam 343 b at which to iteratesteps 609, 611 after having performed the previous iteration of steps609, 611 at the initial beam 343 a at the initial angle 345 a. In anembodiment, step 615 involves a determination of the subsequent angle345 b and angle increment 350 a between the initial angle 345 a and thesubsequent angle 345 b. In one embodiment, the subsequent angle 345 b isbased on the initial angle 345 a, the maximum scan rate at the angle 345a determined in step 609 and the minimum integration time at the angle345 a determined in step 611. In an example embodiment, the subsequentangle θ_(s) is based on the initial angle θ_(i), maximum scan rate S_(m)and minimum integration time I_(m) using:θ_(s)=θ_(i) +S _(m) I _(m)  (5)In an example embodiment, if the initial angle 345 a is −15 degrees, themaximum scan rate is 15 degrees per second and the minimum integrationtime is 2 μs, then the subsequent angle 345 b is about −14.97 degreesusing Equation 5. After determining the subsequent angle in step 615,the method proceeds back to block 609 so that steps 609, 611 areiterated at the subsequent angle.

In step 617, after it is determined that no further iterations of steps609, 611 need to be performed, a scan pattern of the LIDAR system isdefined based on the maximum scan rate from step 609 and the minimumintegration time from step 611 at each angle in the angle range 326. Inone embodiment, the scan pattern includes the maximum scan pattern andminimum integration time for each angle in the angle range 326 betweenthe first angle of the first beam 342 and the second angle of the secondbeam 346. In an example embodiment, the scan pattern is stored in amemory (e.g. memory 704) of the processing system 250. In anotherexample embodiment, the angle increments between adjacent angles in thescan pattern is determined in step 615, e.g. the angle increment is thespacing between the subsequent angle and the initial angle in step 615.In another example embodiment, FIG. 3C depicts an angle increment 350 abetween a subsequent angle 345 b and an initial angle 345 a for purposesof step 615 and the scan pattern determined in step 617.

In step 619, the LIDAR system is operated according to the scan patterndetermined in step 617. In an embodiment, in step 619 the beam of theLIDAR system is scanned in the angle range 326 over one or more cycles,where the scan rate of the beam at each angle is the maximum scan ratein the scan pattern for that angle and the integration time of the LIDARsystem at each angle is the minimum integration time for that angle. Inone embodiment, in step 619, at each angle the processing system 250 ofthe LIDAR system transmits one or more signals to the scanning optics218 so that the scan rate at each angle is the maximum scan rate of thescan pattern for that angle. Additionally, in one embodiment, in step619, at each angle the processing system 250 of the LIDAR system adjuststhe integration time of the acquisition system 240 and/or processingsystem 250 for the return beam 291 received at each angle so that theintegration time is the minimum integration time of the scan pattern forthat angle. This advantageously ensures that the beam is scanned at themaximum scan rate and that the return beam is processed at the shortestintegration time at each angle, while ensuring the LIDAR systemmaintains an adequate SNR at each angle.

FIG. 5B is a graph that illustrates an example of integration time overthe angle range 326 in the system of FIG. 2D, according to anembodiment. The horizontal axis 520 indicates measurement number and thevertical axis 522 indicates integration time in units of seconds (s)times 10⁻⁶ (i.e., μs). Trace 530 depicts the integration time of theLIDAR system during the operation of the LIDAR system in step 619.Measurement region 524 of the trace 530 indicates smaller integrationtimes when the beam is oriented toward the surface 349 (e.g. first angleof first beam 342). Region 526 of the trace 530 indicates longerintegration times when the beam is oriented about parallel to thesurface 349 (e.g. intermediate beam 344). Region 528 of the trace 530indicates smaller integration times when the beam is oriented toward theceiling 347 (e.g. second angle of second beam 346).

FIG. 5C is a graph that illustrates an example of the vertical angleover time over multiple angle ranges in the system of FIG. 2D, accordingto an embodiment. The horizontal axis 502 is time in units of seconds(s) and the vertical axis is angle in units of radians (rad). Trace 540depicts the angle of the beam over time during the scanning over thebeam during multiple scan patterns in step 619. A slope of the curve 540at an instant in time indicates the scan rate of the beam at that time.Region 542 of the trace 540 demonstrates a faster scan rate (e.g. highslope of the curve 540) when the beam is oriented towards the ceiling347; region 544 of the trace 540 also demonstrates a faster scan rate(e.g. high slope of trace 540) when the beam is oriented towards thesurface 349; and region 546 of the trace 540 demonstrates a slower scanrate (e.g. lower slope of the curve 540) when the beam is oriented aboutparallel with the surface 349.

In another embodiment, during or after step 619, the processor operatesthe vehicle 310 based at least in part on the data collected by theLIDAR system during step 619. In one embodiment, the processing system250 of the LIDAR system and/or the processor 314 of the vehicle 310transmit one or more signals to the steering and/or braking system ofthe vehicle based on the data collected by the LIDAR system in step 619.In one example embodiment, the processing system 250 transmits one ormore signals to the steering or braking system of the vehicle 310 tocontrol a position of the vehicle 310 in response to the LIDAR data. Inother embodiments, the processing system 250 transmits one or moresignals to the processor 314 of the vehicle 310 based on the LIDAR datacollected in step 619 and the processor 314 in turn transmits one ormore signals to the steering and braking system of the vehicle 310.

7. Computational Hardware Overview

FIG. 7 is a block diagram that illustrates a computer system 700 uponwhich an embodiment of the invention may be implemented. Computer system700 includes a communication mechanism such as a bus 710 for passinginformation between other internal and external components of thecomputer system 700. Information is represented as physical signals of ameasurable phenomenon, typically electric voltages, but including, inother embodiments, such phenomena as magnetic, electromagnetic,pressure, chemical, molecular atomic and quantum interactions. Forexample, north and south magnetic fields, or a zero and non-zeroelectric voltage, represent two states (0, 1) of a binary digit (bit).Other phenomena can represent digits of a higher base. A superpositionof multiple simultaneous quantum states before measurement represents aquantum bit (qubit). A sequence of one or more digits constitutesdigital data that is used to represent a number or code for a character.In some embodiments, information called analog data is represented by anear continuum of measurable values within a particular range. Computersystem 700, or a portion thereof, constitutes a means for performing oneor more steps of one or more methods described herein.

A sequence of binary digits constitutes digital data that is used torepresent a number or code for a character. A bus 710 includes manyparallel conductors of information so that information is transferredquickly among devices coupled to the bus 710. One or more processors 702for processing information are coupled with the bus 710. A processor 702performs a set of operations on information. The set of operationsinclude bringing information in from the bus 710 and placing informationon the bus 710. The set of operations also typically include comparingtwo or more units of information, shifting positions of units ofinformation, and combining two or more units of information, such as byaddition or multiplication. A sequence of operations to be executed bythe processor 702 constitutes computer instructions.

Computer system 700 also includes a memory 704 coupled to bus 710. Thememory 704, such as a random access memory (RAM) or other dynamicstorage device, stores information including computer instructions.Dynamic memory allows information stored therein to be changed by thecomputer system 700. RAM allows a unit of information stored at alocation called a memory address to be stored and retrievedindependently of information at neighboring addresses. The memory 704 isalso used by the processor 702 to store temporary values duringexecution of computer instructions. The computer system 700 alsoincludes a read only memory (ROM) 706 or other static storage devicecoupled to the bus 710 for storing static information, includinginstructions, that is not changed by the computer system 700. Alsocoupled to bus 710 is a non-volatile (persistent) storage device 708,such as a magnetic disk or optical disk, for storing information,including instructions, that persists even when the computer system 700is turned off or otherwise loses power.

Information, including instructions, is provided to the bus 710 for useby the processor from an external input device 712, such as a keyboardcontaining alphanumeric keys operated by a human user, or a sensor. Asensor detects conditions in its vicinity and transforms thosedetections into signals compatible with the signals used to representinformation in computer system 700. Other external devices coupled tobus 710, used primarily for interacting with humans, include a displaydevice 714, such as a cathode ray tube (CRT) or a liquid crystal display(LCD), for presenting images, and a pointing device 716, such as a mouseor a trackball or cursor direction keys, for controlling a position of asmall cursor image presented on the display 714 and issuing commandsassociated with graphical elements presented on the display 714.

In the illustrated embodiment, special purpose hardware, such as anapplication specific integrated circuit (IC) 720, is coupled to bus 710.The special purpose hardware is configured to perform operations notperformed by processor 702 quickly enough for special purposes. Examplesof application specific ICs include graphics accelerator cards forgenerating images for display 714, cryptographic boards for encryptingand decrypting messages sent over a network, speech recognition, andinterfaces to special external devices, such as robotic arms and medicalscanning equipment that repeatedly perform some complex sequence ofoperations that are more efficiently implemented in hardware.

Computer system 700 also includes one or more instances of acommunications interface 770 coupled to bus 710. Communication interface770 provides a two-way communication coupling to a variety of externaldevices that operate with their own processors, such as printers,scanners and external disks. In general the coupling is with a networklink 778 that is connected to a local network 780 to which a variety ofexternal devices with their own processors are connected. For example,communication interface 770 may be a parallel port or a serial port or auniversal serial bus (USB) port on a personal computer. In someembodiments, communications interface 770 is an integrated servicesdigital network (ISDN) card or a digital subscriber line (DSL) card or atelephone modem that provides an information communication connection toa corresponding type of telephone line. In some embodiments, acommunication interface 770 is a cable modem that converts signals onbus 710 into signals for a communication connection over a coaxial cableor into optical signals for a communication connection over a fiberoptic cable. As another example, communications interface 770 may be alocal area network (LAN) card to provide a data communication connectionto a compatible LAN, such as Ethernet. Wireless links may also beimplemented. Carrier waves, such as acoustic waves and electromagneticwaves, including radio, optical and infrared waves travel through spacewithout wires or cables. Signals include man-made variations inamplitude, frequency, phase, polarization or other physical propertiesof carrier waves. For wireless links, the communications interface 770sends and receives electrical, acoustic or electromagnetic signals,including infrared and optical signals, that carry information streams,such as digital data.

The term computer-readable medium is used herein to refer to any mediumthat participates in providing information to processor 702, includinginstructions for execution. Such a medium may take many forms,including, but not limited to, non-volatile media, volatile media andtransmission media. Non-volatile media include, for example, optical ormagnetic disks, such as storage device 708. Volatile media include, forexample, dynamic memory 704. Transmission media include, for example,coaxial cables, copper wire, fiber optic cables, and waves that travelthrough space without wires or cables, such as acoustic waves andelectromagnetic waves, including radio, optical and infrared waves. Theterm computer-readable storage medium is used herein to refer to anymedium that participates in providing information to processor 702,except for transmission media.

Common forms of computer-readable media include, for example, a floppydisk, a flexible disk, a hard disk, a magnetic tape, or any othermagnetic medium, a compact disk ROM (CD-ROM), a digital video disk (DVD)or any other optical medium, punch cards, paper tape, or any otherphysical medium with patterns of holes, a RAM, a programmable ROM(PROM), an erasable PROM (EPROM), a FLASH-EPROM, or any other memorychip or cartridge, a carrier wave, or any other medium from which acomputer can read. The term non-transitory computer-readable storagemedium is used herein to refer to any medium that participates inproviding information to processor 702, except for carrier waves andother signals.

Logic encoded in one or more tangible media includes one or both ofprocessor instructions on a computer-readable storage media and specialpurpose hardware, such as ASIC 720.

Network link 778 typically provides information communication throughone or more networks to other devices that use or process theinformation. For example, network link 778 may provide a connectionthrough local network 780 to a host computer 782 or to equipment 784operated by an Internet Service Provider (ISP). ISP equipment 784 inturn provides data communication services through the public, world-widepacket-switching communication network of networks now commonly referredto as the Internet 790. A computer called a server 792 connected to theInternet provides a service in response to information received over theInternet. For example, server 792 provides information representingvideo data for presentation at display 714.

The invention is related to the use of computer system 700 forimplementing the techniques described herein. According to oneembodiment of the invention, those techniques are performed by computersystem 700 in response to processor 702 executing one or more sequencesof one or more instructions contained in memory 704. Such instructions,also called software and program code, may be read into memory 704 fromanother computer-readable medium such as storage device 708. Executionof the sequences of instructions contained in memory 704 causesprocessor 702 to perform the method steps described herein. Inalternative embodiments, hardware, such as application specificintegrated circuit 720, may be used in place of or in combination withsoftware to implement the invention. Thus, embodiments of the inventionare not limited to any specific combination of hardware and software.

The signals transmitted over network link 778 and other networks throughcommunications interface 770, carry information to and from computersystem 700. Computer system 700 can send and receive information,including program code, through the networks 780, 790 among others,through network link 778 and communications interface 770. In an exampleusing the Internet 790, a server 792 transmits program code for aparticular application, requested by a message sent from computer 700,through Internet 790, ISP equipment 784, local network 780 andcommunications interface 770. The received code may be executed byprocessor 702 as it is received, or may be stored in storage device 708or other non-volatile storage for later execution, or both. In thismanner, computer system 700 may obtain application program code in theform of a signal on a carrier wave.

Various forms of computer readable media may be involved in carrying oneor more sequence of instructions or data or both to processor 702 forexecution. For example, instructions and data may initially be carriedon a magnetic disk of a remote computer such as host 782. The remotecomputer loads the instructions and data into its dynamic memory andsends the instructions and data over a telephone line using a modem. Amodem local to the computer system 700 receives the instructions anddata on a telephone line and uses an infra-red transmitter to convertthe instructions and data to a signal on an infra-red a carrier waveserving as the network link 778. An infrared detector serving ascommunications interface 770 receives the instructions and data carriedin the infrared signal and places information representing theinstructions and data onto bus 710. Bus 710 carries the information tomemory 704 from which processor 702 retrieves and executes theinstructions using some of the data sent with the instructions. Theinstructions and data received in memory 704 may optionally be stored onstorage device 708, either before or after execution by the processor702.

FIG. 8 illustrates a chip set 800 upon which an embodiment of theinvention may be implemented. Chip set 800 is programmed to perform oneor more steps of a method described herein and includes, for instance,the processor and memory components described with respect to FIG. 7incorporated in one or more physical packages (e.g., chips). By way ofexample, a physical package includes an arrangement of one or morematerials, components, and/or wires on a structural assembly (e.g., abaseboard) to provide one or more characteristics such as physicalstrength, conservation of size, and/or limitation of electricalinteraction. It is contemplated that in certain embodiments the chip setcan be implemented in a single chip. Chip set 800, or a portion thereof,constitutes a means for performing one or more steps of a methoddescribed herein.

In one embodiment, the chip set 800 includes a communication mechanismsuch as a bus 801 for passing information among the components of thechip set 800. A processor 803 has connectivity to the bus 801 to executeinstructions and process information stored in, for example, a memory805. The processor 803 may include one or more processing cores witheach core configured to perform independently. A multi-core processorenables multiprocessing within a single physical package. Examples of amulti-core processor include two, four, eight, or greater numbers ofprocessing cores. Alternatively or in addition, the processor 803 mayinclude one or more microprocessors configured in tandem via the bus 801to enable independent execution of instructions, pipelining, andmultithreading. The processor 803 may also be accompanied with one ormore specialized components to perform certain processing functions andtasks such as one or more digital signal processors (DSP) 807, or one ormore application-specific integrated circuits (ASIC) 809. A DSP 807typically is configured to process real-world signals (e.g., sound) inreal time independently of the processor 803. Similarly, an ASIC 809 canbe configured to performed specialized functions not easily performed bya general purposed processor. Other specialized components to aid inperforming the inventive functions described herein include one or morefield programmable gate arrays (FPGA) (not shown), one or morecontrollers (not shown), or one or more other special-purpose computerchips.

The processor 803 and accompanying components have connectivity to thememory 805 via the bus 801. The memory 805 includes both dynamic memory(e.g., RAM, magnetic disk, writable optical disk, etc.) and staticmemory (e.g., ROM, CD-ROM, etc.) for storing executable instructionsthat when executed perform one or more steps of a method describedherein. The memory 805 also stores the data associated with or generatedby the execution of one or more steps of the methods described herein.

8. Alterations, Extensions and Modifications

In the foregoing specification, the invention has been described withreference to specific embodiments thereof. It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the invention. Thespecification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense. Throughout thisspecification and the claims, unless the context requires otherwise, theword “comprise” and its variations, such as “comprises” and“comprising,” will be understood to imply the inclusion of a stateditem, element or step or group of items, elements or steps but not theexclusion of any other item, element or step or group of items, elementsor steps. Furthermore, the indefinite article “a” or “an” is meant toindicate one or more of the item, element or step modified by thearticle.

9. References

The following references were cited herein, the entire contents of eachof which are hereby incorporated by reference as if fully set forthherein, except for terminology inconsistent with that used herein.

What is claimed is:
 1. A light detection and ranging (LIDAR) system,comprising: one or more scanning optics configured to transmit atransmit signal at a scan rate over an angle range defined by a firstangle and a second angle; and one or more processors configured to:receive a first signal-to-noise ratio (SNR) value associated with thescan rate responsive to operation of the one or more scanning optics;receive a second SNR value associated with an integration time ofprocessing a return signal received responsive to operation of the oneor more scanning optics; identify a particular design target range for aparticular angle of the angle range; determine a particular scan rate,for the particular angle and using the particular design target range,responsive to comparing the first SNR value to an SNR threshold;determine an integration time, for the particular angle, responsive tocomparing the second SNR value to the SNR threshold; determine a scanpattern using the particular scan rate and the integration time; andcontrol operation of the one or more scanning optics using the scanpattern to determine a range to the target.
 2. The LIDAR system of claim1, further comprising a detector array that receives the return signal.3. The LIDAR system of claim 1, further comprising: a laser source; anoptical waveguide coupled with the laser source; and a collimation opticconfigured to receive an optical signal from the optical waveguide andshape the optical signal into a target beam, the one or more scanningoptics configured to receive the target beam from the collimation opticto transmit the transmit signal at the scan rate responsive to receivingthe target beam.
 4. The LIDAR system of claim 3, wherein the collimationoptic focuses the return signal to a tip of the optical waveguide. 5.The LIDAR system of claim 1, wherein the first SNR value is of aplurality of first SNR values, the second SNR value is of a plurality ofsecond SNR values, and the one or more processors are configured to:determine the particular scan rate responsive to comparing the pluralityof first SNR values to the SNR threshold; and determine the integrationtime responsive to comparing the plurality of second SNR values to theSNR threshold.
 6. The LIDAR system of claim 1, wherein the first angleis in a range from about negative 25 degrees to about negative 10degrees with respect to a reference point, and the second angle is in arange from about 10 degrees to about 20 degrees with respect to thereference point, wherein respective values of the particular designtarget range at the first angle and the second angle are less than about15 meters and a value of the particular design target range between thefirst angle and the second angle is greater than about 100 meters. 7.The LIDAR system of claim 1, wherein the one or more processors areconfigured to determine the particular scan rate to limit a walkoffdistance between a first location from which the transmit signal istransmitted and a second location at which the return signal isreceived.
 8. The LIDAR system of claim 1, wherein the one or moreprocessors are configured to determine at least one of: the particulardesign target range to be less at the first angle and the second anglethan at an angle between the first angle and the second angle; theparticular scan rate to be greater at the first angle and the secondangle than at the angle between the first angle and the second angle; orthe integration time to be shorter at the first angle and the secondangle than at the angle between the first angle and the second angle. 9.The LIDAR system of claim 1, wherein the one or more processors areconfigured to control operation of an autonomous vehicle using the rangeto the target.
 10. The LIDAR system of claim 1, wherein: the particulardesign target range is a maximum design target range for the particularangle of the angle range; the particular scan rate is a maximum scanrate for the particular angle of the angle range; and the integrationtime is a minimum integration time for the particular angle of the anglerange.
 11. A method, comprising: receiving a first signal-to-noise ratio(SNR) value associated with a scan rate at which one or more scanningoptics transmit a transmit signal over an angle range defined by a firstangle and a second angle; receiving a second SNR value associated withan integration time of processing a return signal received responsive tooperation of the one or more scanning optics; identifying a particulardesign target range for a particular angle of the angle range;determining a particular scan rate, for the particular angle and usingthe particular design target range, responsive to comparing the firstSNR value to an SNR threshold; determining a particular integrationtime, for the particular angle, responsive to comparing the second SNRvalue to the SNR threshold; determining a scan pattern using theparticular scan rate and the particular integration time; andcontrolling operation of the one or more scanning optics using the scanpattern to determine a range to the target.
 12. The method of claim 11,wherein the first SNR value is of a plurality of first SNR values, thesecond SNR value is of a plurality of second SNR values, the methodfurther comprising: determining the particular scan rate responsive tocomparing the plurality of first SNR values to the SNR threshold; anddetermining the integration time responsive to comparing the pluralityof second SNR values to the SNR threshold.
 13. The method of claim 11,wherein: the first angle is in a range from about negative 25 degrees toabout negative 10 degrees with respect to a reference point, and thesecond angle is in a range from about 10 degrees to about 20 degreeswith respect to the reference point; and respective values of theparticular design target range at the first angle and the second angleare less than about 15 meters and a value of the particular designtarget range between the first angle and the second angle is greaterthan about 100 meters.
 14. The method of claim 11, wherein determiningthe particular scan rate comprises determining the particular scan rateto limit a walkoff distance between a first location from which thetransmit signal is transmitted and a second location at which the returnsignal is received.
 15. The method of claim 11, comprising determiningat least one of: the particular design target range to be less at thefirst angle and the second angle than at an angle between the firstangle and the second angle; the particular scan rate to be greater atthe first angle and the second angle than at the angle between the firstangle and the second angle; or the integration time to be shorter at thefirst angle and the second angle than at the angle between the firstangle and the second angle.
 16. The method of claim 11, furthercomprising controlling operation of an autonomous vehicle using therange to the target.
 17. An autonomous vehicle control system,comprising: a LIDAR sensor configured to: receive a firstsignal-to-noise ratio (SNR) value associated with a scan rate at whichone or more scanning optics transmit a transmit signal over an anglerange defined by a first angle and a second angle; receive a second SNRvalue associated with an integration time of processing a return signalreceived responsive to operation of the one or more scanning optics;identify a particular design target range for a particular angle of theangle range; determine a particular scan rate, for the particular angleand using the particular design target range, responsive to comparingthe first SNR value to an SNR threshold; determine an integration time,for the particular angle, responsive to comparing the second SNR valueto the SNR threshold; determine a scan pattern using the particular scanrate and the integration time; and control operation of the one or morescanning optics using the scan pattern to determine a range to thetarget; and a vehicle controller configured to control operation of anautonomous vehicle using the range to the target.
 18. The autonomousvehicle control system of claim 17, wherein the first SNR value is of aplurality of first SNR values, the second SNR value is of a plurality ofsecond SNR values, and the LIDAR sensor is configured to: determine thescan rate responsive to comparing the plurality of first SNR values tothe mnimum SNR threshold; and determine the integration time responsiveto comparing the plurality of second SNR values to the SNR threshold.19. The autonomous vehicle control system of claim 17, wherein the firstangle is in a range from about negative 25 degrees to about negative 10degrees with respect to a reference point, and the second angle is in arange from about 10 degrees to about 20 degrees with respect to thereference point, wherein respective values of the particular designtarget range at the first angle and the second angle are less than about15 meters and a value of the design target range between the first angleand the second angle is greater than about 100 meters.
 20. Theautonomous vehicle control system of claim 17, wherein the LIDAR sensoris configured to determine the particular scan rate to limit a walkoffdistance between a first location from which the transmit signal istransmitted and a second location at which the return signal isreceived.